Conversion of olefins to gasoline and/or distillate products in disclosed in U.S. Pat. Nos. 3,960,978 and 4,021,502 (Givens, Plank and Rosinski) wherein gaseous olefins in the range of ethylene to pentene, either alone or in admixture with paraffins are converted into an olefinic gasoline blending stock by contacting the olefins with a catalyst bed made up of ZSM-5 or releated zeolite. In U.S. Pat. Nos. 4,150,062 and 4,227,992 Garwood et al disclose the operating conditions for the Mobil Olefin to Gasoline Distillate (MOGD) process for selective conversion of C.sub.3 + olefins.
The phenomena of shape-selective polymerization are discussed by Garwood in ACS Symposium Series No. 218, Intrazeolite Chemistry, "Conversion of C.sub.2 -C.sub.10 to Higher Olefins over Synthetic Zeolite ZSM-5", 1983 American Chemical Society.
Typically, the process recycles cooled light hydrocarbons from a high-temperature, high-pressure separator downstream of the catalyst bed back into the reaction zone where additional olefins are converted to gasoline and distillate products. If the reaction of the olefins in converting them to distillate and gasoline is allowed to progress in the catalyst stream without any measure taken to prevent the accumulation of heat, the reaction becomes so exothermically accelerated as to result in high temperatures and the production of undesired products.
In the process for catalytic conversion of olefins to heavier hydrocarbons by catalytic oligomerization using an acid crystaline zeolite, such as ZSM-5 or related shape-selective catalyst, process conditions can be varied to favor the formation of either gasoline or distillate range products. At moderate temperature and relatively high pressure, the conversion conditions favor distillate range product having a normal boiling point of at least 165.degree. C. (330.degree. F.). Lower olefinic feedstocks containing C.sub.2 -C.sub.6 alkenes may be converted selectively; however, the distillate mode conditions do not convert a major fraction of ethylene due to low severity conditions. While propene, butene-1 and others may be converted to the extent of 50 to 95% in the distillate mode, only about 10 to 50% of the ethylene component will be consumed.
In the high severity or gasoline mode, ethylene and the other lower olefins are catalytically oligomerized at higher teperature and moderate pressure. Under these conditions ethylene conversion rate is greatly increased and lower olefin oligomerization is nearly complete to produce an olefinic gasoline comprising hexene, heptene, octene and other C.sub.6 + hydrocarbons in good yield. To avoid excessive temperatures in the exothermic reactors, the lower olefinic feed may be diluted. In the distillate mode operation, olefinic gasoline may be recycled and further oligomerized, as disclosed in U.S. Pat. Nos. 4,211,640 (Garwood and Lee) and 4,433,185 (Tabak). The above cited publications are incorporated herein by reference.
One important source of olefinic feedstocks of interest for conversion to heavier fuel products is the intermediate olefin-rich light oil or naphtha obtained from Fischer-Tropsch conversion of synthesis gas. However, these symbol materials contain, in addition to olefins, a minor amount of co-produced oxygenated hydrocarbons. It has been found that these oxygenates can interfere with catalytic oligomerization of olefins, particularly under the low severity conditions employed for making distillate and heavier hydrocarbons. It is an object of this invention to overcome catalytic deactivation by oxygenates during oligomerization by the MOGD process by extracting these non-hydrocarbon impurities from the olefinic stream prior to oligomerization.
Commonly assigned U.S. Pat. No. 4,513,156 and U.S. Ser. No. 716,317 filed Mar. 27, 1985 disclose improvements in oligomerizing olefinic Fischer-Tropsch liquids. The improvement comprises extracting oxygenates from the feedstock with a polar solvent such as water; converting the extracted feedstock in a primary stage oligomerization reactor; recovering the oxygenates and reacting them with the light hydrocarbons from the primary stage for conversion in a second stage reactor for converting the oxygenates and light hydrocarbons to heavier hydrocarbons.
U.S. Pat. No. 2,918,486 discloses the extraction of alcohols from olefine hydrocarbons with propylene carbonate. Among the olefin streams which can be treated include those obtained by the Fischer-Tropsch process. The patentee states that the process is selective for extracting alcohols in preference to organic carbonyl compounds such as aldehydes and ketones. The patent further discloses recovering dissolved solvent from the raffinate by washing the raffinate with water. Examples in the specification are limited to extracting C.sub.6 + fractions from a Fischer-Tropsch process.
U.S. Pat. No. 3,305,592 discloses the separation of normal primary alkanols from admixtures thereof with olefins utilizing liquid-liquid extraction with a polar oxygen-containing solvent such as furfural, benzol alcohol, dimethylformamide and furfural alcohol.
U.S. Pat. No. 3,489,462 discloses selective solvent extraction of hydrocarbon streams using a solvent mixture comprising furfural and one or more ketones.
U.S. Pat. No. 3,565,795 discloses a process for extracting aromatic compounds from a petroleum distillate fraction by contacting the distillate fraction with hydroxyl substituted aliphatic ketones.
U.S. Pat. No. 3,663,641 discloses removing oxygenated materials from unsaturated compounds such as butadiene by a liquid-liquid phase water wash at a temperature below about 32.degree. F. Freezing point depressants, such as alcohol, glycol, etc. can be added to avoid ice formation.